Geoscience Reference
In-Depth Information
Floods and flood control
HUMAN IMPACT
Flooding is the inundation of land beyond the normal confines of a channel or coastline, either by overflow of excess
water or its influx via shallow subsurface or low-lying routes. Coastal flooding through abnormal tidal surges or waves,
often coinciding with storms and high river discharge, is covered in
Chapter 17.
All other forms of flooding involve
water temporarily unable to enter a channel, in the case of
depression storage
and sheetflow, or re-enter it after
an
overbank discharge
. Floods originate from extreme meteorological events and their indirect and geophysical
consequences (
Figure 14.11
).
Flooding is the most frequent and widespread form of 'natural' disaster, affecting more
humans than any other physical hazard. It is on the increase in many regions through climate change yet so far takes
a surprisingly small toll of human life, although this is not the enigma it may seem. However, that may very well
change if IPCC 2007 predictions for the twenty-first century are fulfilled. Global warming steers two very broad trends
in global precipitation and river flow, towards increased precipitation (mid to high latitudes) or increased drought (low
to Mediterranean latitudes) but with greater intensities and flash-flood risks in both regimes. More than 20 per cent
of global populations live in catchments very likely to experience increased flooding by the 2080s.
Flooding is so common that people who live in flood-susceptible areas rarely do so unawares. Most major cities built
on flood plains are channelling river discharge between embankment 'landscapes' to counter the very real risk of
overtopping (
Plate 14.5
).
Risk is accepted either by choice, in pursuit of economic or aesthetic gain, or of necessity
because of the pressure on, or the particular attributes of, the land. Historically the latter have welcomed flooding
itself as a means of natural irrigation and nutrient replenishment (e.g. in farmland along the Nile). Risk assessments
of varying degrees of formality are undertaken and means of evading, avoiding or mitigating flood losses are developed.
Unfortunately, human occupation of floodable land - and other activities elsewhere in the catchment - usually increase
the frequency and intensity of flood hazard (
Figure 14.12
and box on p.333). Overbank flooding is an undesirable
discharge level and its inevitability is seen from single event or annual hydrographs. The 'area under the curve' of
rising and receding limbs represents the total discharge generated by input. The shorter the lag time, the steeper
the rising limb, the higher the PDV and the more likely that any particular flood threshold will be reached. The curve
is thus 'elastic', and human activity in the catchment can be assessed in terms of whether it reduces lag times and
enhances flood risk or extends lag times and thereby reduces it.
Flood adjustment measures commence with prediction of the scale and frequency of future floods. Assessment of
historical records and future forecasting generates the probable
recurrence interval
of any particular critical
discharge, or the mean annual flood or maximum probable flood. The latter, worst-case, scenario assumes that the
atmosphere 'dumps' all available moisture in a single event and that antecedentcatchment conditions are favourable.
Prediction permits measures to be taken to reduce flood risk but the ability to do so depends on the technological
status and economic wealth of the population. Knowledge of recurrence intervals alone is not enough, and the
response depends on ability to forecast the next event of a particular size within the return period from the monitoring
of catchment conditions and weather forecasts. There are four principal forms of flood protection or mitigation. Hard
engineering options are most likely in urban or industrial areas and are covered in the box on p.333. Soft options
permit the flood to develop but, through a warning system, evacuate people and partially shut down or protect
installations to reduce losses. Structures can fail or be bypassed, leading to more serious if less frequent flooding.
Temporary barriers are a developing option where hard structures may be difficult or too expensive to engineer and
they too rely on good flood warning schemes (
Plate 14.6
)
Passive options do nothing to reduce lag times or raise
critical thresholds but sustain economic damage as a less expensive alternative.
